English question, please help?

No, not in that sentence. You could also say:

As a student, he's very obedient.
He's a very obedient student.

Phantom, it's perfectly acceptable for you to put a comma before 'who'; it changes the sentence from a standard desciption of the student, to a rather subtle and amusing commentary on that particular student. I like it!!

(A comma, as well as being a 'divider' of points in a list, etc, also acts as a 'pause mechanism' ... when you're reading out loud, if you would naturally take a breath or pause, that calls for a comma. In your sentence, the addition of the comma is ironic - great!)